Surge protection apparatus with improved circuit for reliable sparkover

ABSTRACT

Surge protection apparatus such as a lightning arrester is provided with a plurality of serially connected, power-handling spark gaps with a network of voltage-grading capacitors connected thereacross. A control spark gap is connected across one of the power-handing spark gaps and has a controlled and reliable sparkover characteristic. In circuit with the control spark gap are inductive and capacitive circuit elements to assure sparkover of the entire series string of power-handling spark gaps quickly upon sparkover of the control spark gap while avoiding damage to the control spark gap from excessive current flow.

United States Patent Inventors Appl. No.

Filed Patented Assignee Joseph C. Osterhout;

Richard E. Kenn/on, both 0! Blooinlngton, Ind.

June 30, 1970 Oct. 5, 197 1 Westinghouse Electric Corporation Pittsburgh, Pa.

SURGE PROTECTION APPARATUS WITH IMPROVED CIRCUIT FOR RELIABLE SPARKOVER 7 Clalma, 1 Drawing Fig.

U.S. C1 317/70,

315/36. 317/61. 317/68 Int. Cl 02h 9/06 Field of Search 317/70, 61

[56] References Clted UNITED STATES PATENTS 2,862,152 11/1958 Ryden 317/16 X 3,418,530 12/1968 Cheever 317/70 X 3,510,726 5/1970 Harder 315/36 X Primary Examiner-James D. Trammell ArtorneysA. T. Stratton, Gordon H. Telfer and F. P. Lyle SURGE PROTECT ION APPARATUS WITH IMPROVED CIRCUIT FOR RELIABLE SPARKOVER BACKGROUND OF THE INVENTION l. Field of the Invention This invention is directed to electrical surge protection apparatus, such as lightning arresters, and particularly to lightning arresters including among the spark gap elements thereof a control spark gap element to fire at a relatively precise potential level to assure sparkover of the entire lightning arrester.

2. Description ofthe Prior Art In accordance with the teachings of copending application Ser. No. 679,315, new U.S. Pat. No. 3,5l0,726, filed Oct. 31, I967, by J. E. Harder and assigned to the assignee of the present invention, it is known to provide a lightning arrester with a series string of spark gaps of which at least one, but no more than a few, are accurately calibrated control gaps with voltage-grading capacitors alternately connected across adjacent pairs of spark gaps. This arrangement ensures a precise, reproduction sparkover level for the initiation of breakdown of the series string of spark gaps. This permits a lightning arrester in which typically only one of the spark gap elements need to be precisely fabricated to provide a controlled protective level, thus minimizing cost while achieving good accuracy and reliability.

In accordance with the description contained in copending application Ser. No. 765,241, now abandoned, filed Oct. 4, I968, by H. Riemersma and A. M. Sletten and assigned to the assignee of the present invention, of which application Ser. No. 89,093, filed Nov. [2, 1970, is a continuation, it is known to provide an arrester with spark gaps that include a control spark gap in accordance with the above-mentioned Harder application but having in parallel therewith a power-handling spark gap, so that after the initiation of sparkover the current is carried by the power-handling spark gap and the control spark gap is not subject to problems such as electrode erosion that would impair subsequent sparkover at the same potential level.

Additionally, the latter copending application discloses a circuit arrangement in which the control spark gap is connected to the string of serially connected power-handling spark gaps through a resistance element serving as a current limiter to prevent the normal arrester current from passing through the control spark gap element to avoid damage to that element. Also the control spark gap is connected through another resistance element to the voltage grading capacitors that are associated therewith. The latter resistance element is to prevent excessive discharge current through the voltage grading capacitor that is across the control spark gap. It is found that such a circuit ix difficult to optimize because of contradictory requirements imposed upon the resistance ele ments. It is desirable that they be large enough to limit current in accordance with the purposes mentioned above, and yet is is also desirable that the circuit branch in which the control spark gap is located act as a direct short upon firing to impose as much voltage as possible across the remaining spark gaps.

SUMMARY OF THE INVENTION The invention provides a lightning arrester with a circuit associated with a control spark gap element that assures rapid sparkover of each of the power-handling spark gaps upon sparkover of the control spark gap and also avoids large current passing through the control spark gap for any appreciable time that could cause damage thereto.

The circuit of the invention includes with the control spark gap inductive and capacitive impedance means, with negligible resistance in series with the control spark gap, so that upon sparkover of the control spark gap an alternating waveform of voltage is established in the control spark gap circuit branch that has the effect of increasing the voltage across an adjacent power-handling spark gap up to three times the otherwise present voltage and to ensure reliable sparkover. It has been found by study of the prior art arrangements mentioned above that the principal failure has been that the next adjacent spark gap (efl'ectively in series with the control gap) has failed to sparkover due to insufficient voltage.

The required elements are essentially an inductance and a capacitance in a circuit arrangement to provide a characteristic resonance that will produce a standing waveform of the current and voltage. Any substantial series resistance that would cut down the voltage applied across the adjacent spark gap element is avoided. A resistance element is, however, provided in parallel connection across the capacitor so as to provide a discharge path that in effect resets the lightning arrester for subsequent tiring.

The voltage swing produced by the inductive and capacitive elements results in a marked increase in voltage across the adjacent spark gap. The capacitor, however, prevents large power currents through the control spark gap, where the capacitive and inductive impedance elements are in series.

BRIEF DESCRIPTION OF THE DRAWING The single FIG. is a circuit schematic of one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, an exemplary embodiment of ap plicants' invention is illustrated. The circuit is shown as a plurality of series connections 10, 20, 30 and 40 of elements between a powerline L to be protected and ground.

A first series connection 10 comprises a plurality of powerhandling spark gaps ll, 12, 13, I4 and [5; each such identified spark gap may comprise a plurality of smaller spark gaps. Connected between the power-handling spark gap elements are a plurality of relatively low-value nonlinear currentlimiting resistances or valve blocks 51, 52, 53, 54, S5, 56, 57, and 58.

The circuit also includes second and third series connections 20 and 30 of voltage-grading capacitors 21, 22, 23, 3], 32 and 33 that have alternating, successive connections to the first series connection 10 of the power-handling spark gaps. In this embodiment, capacitors 22 and 23 of the second series connection 20 are each connected across a pair of the powerhandling spark gaps, but the lowermost capacitor 21 is connected only across a single spark gap ll, while in the third series connection 30 capacitors 31 and 32 are shown each connected across a pair of power-handling spark gaps but alternating and overlapping those which capacitors of the second series connection are connected across. The uppermost capacitor 33 is connected across a single spark gap 15. Typically capacitors 21 and 33 should be about twice the value of the other capacitors (e.g. 400 pF and 200 pF, respectively). Reference should be made to copending application Ser. No. 679,3l5 by Harder for further description of this type of stepped-capacitor voltage-grading scheme.

A fourth series connection 40 is provided that includes a plurality of nonlinear voltage dividing resistors 41, 42, 43, 44 and 45, one connected across each of the power-handling spark gaps. The resistors 41 through 45 are typically of substantial magnitude, such as the order of megohms.

One of the voltagegrading capacitors 2i of the second series connection has a circuit branch 60 connected with it that includes a control spark gap 61 so as to provide accurate and reliable firing of the arrester stack. The control spark gap 61 is one that is carefully fabricated and sealed to provide a fixed level of sparkover. One example of such a control spark gap that may be employed in apparatus in accordance with this in vention is disclosed in the above-mentioned copending application Ser. No. 765,241.

The circuit branch 60 is additionally provided with inductive and capacitive impedance elements 62 and 63, respectively. Without the inductive and capacitive impedance elements 62 and 63, sparkover of the control gap 6! may not cause sufficient voltage buildup to produce reliable sparkover of the adjacent series spark gap element 12 and the subsequent cascading of the entire series string of spark gaps. The impedance elements 62 and 73 in accordance with this invention provide a resonant or oscillatory effect on the voltage at point A when control gap 61 sparks over. The voltage at point A will swing between positive and negative maximum values. The effect is a transient one without affecting the absolute voltage at point B or total arrester voltage. Within the first cycle of oscillation, when the voltage at. A reaches its most negative value and the voltage at B remains at some positive value, the voltage drop from A to B, and hence across gap 12, is substantially greater than would occur if the inductive and capacitive impedance elemenm 62 and 63 were not provided in circuit branch 60. Such increased voltage across gap 12 ensures its sparkover and the cascading of the stack.

The time of the maximum voltage reversal at point A can be controlled by the magnitude of the impedance elements to match the time of greatest probability of firing of the adjacent power spark gap l2. For example with an inductance of 40 microhenries and a capacitance of 1,000 picofarads, reversal time is approximately 0.4 microsecond.

There is also included a resistance element 64 across the capacitor 63 so chosen that the discharge of the capacitor 63 occurs in a time that is long compared with the total stack sparkover time, but is short compared to the likely time between required operations. Typically the resistance 64 may be about l0,000 ohms and provide a discharge time for a 1,000 picofarad capacitor 63 of about 100 microseconds.

The capacitor 63 is in series with the inductance 62 and the control gap 6l and thus provides the additional function of preventing excessive current flow through the control spark gap 6|. This function is advantageous because otherwise the control gap can be damaged by high power-follow current. (Resistor 64 is sufficiently large not to conduct excessive current.) Spark gap ll will carry the power-follow current.

Other circuit arrangements can be provided. For example, the resistance 64 can be connected across both the capacitance and inductance in series. Alternatively, a capacitance and inductance may be provided in parallel and reliable sparkover would be achieved but not the same degree of protection of the control spark gap from the power-follow current.

The circuit is merely representative in showing five powerhandling spark gaps; typically a greater number would be employed. especially for station-type arresters. The structural arrangement of the circuit elements may be in accordance with for example, that described in copending application Ser. No. 816,485, now U.S. Pat. No. 3,534,22l, filed Apr. I6, 1969. by R. E. Kennon and assigned to the assignee of the present in vention.

More than one control spark gap may be employed in a lightning arrester embodying the invention and such control spark gaps may be variously positioned throughout the stack.

What is claimed is:

1. An electrical surge arrester comprising: a serially connected plurality of power-handling spark gaps; a first circuit branch electrically connected across at least one of said power-handling spark gaps, said first circuit branch comprising a voltage-grading capacitor; a second circuit branch electrically connected across said at least one power-handling spark gap, said second circuit branch comprising a control spark gap and inductive and capacitive impedance means for assuring rapid breakdown of said plurality of power-handling spark gaps upon breakdown of said control spark gap.

2. The subject matter of claim 1 wherein: said impedance means comprises an inductance element having an inherent resistance and a capacitance element; said inherent resistance of said inductance element being substantially the only re sistance in said first and second circuit branches.

3. The subject matter of claim 1 wherein: said impedance means comprises serially connected inductive and capacitive elements.

4. The subject matter of claim 2 further comprising: a resistance element connected in a capacitor dischar e path across at least said capacitance element of said secon circuit branch.

5. A lightning arrester comprising: a plurality of power-handling spark gaps in a first series connection between two circuit points; a plurality of voltage-grading capacitors in second and third series connections between said two circuit points. and capacitors of said second and third series connections having successively alternating connections to said first series connection of power-handling spark gaps; one of said capacitors of said second and third series connections also being connected in a circuit loop with a control spark gap and inductive and capacitive impedance elements.

6. The subject matter of claim 5 wherein: said inductive and capacitive impedance element are in series connection with said control spark gap.

7. The subject matter of claim 5 further comprising: a fourth series connection between said two circuit points comprising a plurality of resistors, each of the order of megohms. each connected across one of said plurality of power-handling spark gaps; said first series connection including resistors of substantially smaller magnitude than those of said fourth series connection connected between adjacent ones of said plurality of power-handling spark gaps. 

1. An electrical surge arrester comprising: a serially connected plurality of power-handling spark gaps; a first circuit branch electrically connected across at least one of said power-handling spark gaps, said first circuit branch comprising a voltagegrading capacitor; a second circuit branch electrically connected across said at least one power-handling spark gap, said second circuit branch comprising a control spark gap and inductive and capacitive impedance means for assuring rapid breakdown of said plurality of power-handling spark gaps upon breakdown of said control spark gap.
 2. The subject matter of claim 1 wherein: said impedance means comprises an inductance element having an inherent resistance and a capacitance element; said inherent resistance of said inductance element being substantially the only resistance in said first and second circuit branches.
 3. The subject matter of claim 1 wherein: said impedance means comprises serially connected inductive and capacitive elements.
 4. The subject matter of claim 2 further comprising: a resistance element connected in a capacitor discharge path across at least said capacitance element of said second circuit branch.
 5. A lightning arrester comprising: a plurality of power-handling spark gaps in a first series connection between two circuit points; a plurality of voltage-grading capacitors in second and third series connections between said two circuit points, and capacitors of said second and third series connections having successively alternating connections to said first series connection of power-handling spark gaps; one of said capacitors of said second and third series connections also being connected in a circuit loop with a control spark gap and inductive and capacitive impedance elements.
 6. The subject matter of claim 5 wherein: said inductive and capacitive impedance element are in series connection with said control spark gap.
 7. The subject matter of claim 5 further comprising: a fourth series connection between said two circuit points comprising a plurality of resistors, each of the order of megohms, each connected across one of said plurality of power-handling spark gaps; said first series connection including resistors of substantially smaller magnitude than those of said fourth series connection connected between adjacent ones of said plurality of power-handling spark gaps. 